The present invention relates to a semiconductor device having a floating gate formed therein, and a method of fabricating the same.
According to a conventional method of fabricating a semiconductor device having a floating gate formed therein, after a buried trench through which an isolation structure is intended to be buried in a silicon substrate is formed, oxidation is performed for the purpose of restoring an inner wall of the buried trench having an etching damage received thereby. However, there is encountered a problem that even polycrystalline silicon of which a floating gate is made is oxidized, so that a bird's beak occurs in the floating gate.
The occurrence of the bird's beak in the floating gate may cause such problems that shapes of peripheral portions of the floating gate vary, that a short channel effect becomes easy to occur, and that the reliability of the semiconductor device is reduced. In addition, an influence of the bird's beak is further actualized along with the progress of scale down of the circuits.
A technique for preventing crystal grains of silicon from spreading to an isolation structure side due to a heat treatment after completion of processing for a floating gate electrode (floating gate) by forming a nitride film on a sidewall of the floating gate electrode made of polycrystalline silicon or amorphous silicon, especially, on a sidewall of the floating gate electrode along a channel length direction is reported as conventional one. This technique, for example, is described in Japanese Patent KOKAI No. 2004-186185. However, this technique is not developed for the purpose of suppressing the bird's beak, and thus cannot suppress the occurrence of the bird's beak in the floating gate.